In a case where oversteer occurs while a vehicle is turning (i.e. while the vehicle is in a turning state), it is effective to steer a steering operation member (a steering wheel) in a direction opposite to a direction of a yawing motion of the vehicle so as to steer steered wheels in the direction opposite to the direction of the yawing motion in order to stabilize the vehicle by reducing a degree of the oversteer.
In a case where a slip restricting control (which will be hereinafter referred to as a μ split control), such as an anti-skid control (ABS control), a traction control (TCS control) and the like for restricting a slip occurring at a wheel, is executed while the vehicle is traveling on a road surface, in which a friction coefficient of a road surface contacting a left wheel and a friction coefficient of the road surface contacting a right wheel differ from one another (which will be hereinafter referred to as a μ split road surface), a difference (a braking force difference in a case where the ABS control is executed, a driving force difference in a case where the TCS control is executed) is generated between longitudinal forces of the right wheels and the left wheels (i.e. a frictional force generated between the road surface and a tire in an acceleration/deceleration direction, which is also referred to as the braking/driving force). A deflection (yawing motion) may occur at the vehicle because of the longitudinal force difference between the right and left wheels. In order to restrict the deflection of the vehicle and to stabilize the vehicle, it is effective to steer the steering operation member in the direction opposite to the direction of the yawing motion of the vehicle so as to steer the steered wheels in the direction opposite to the direction of the yawing motion.
The above-described operation of steering the steering operation member in the direction opposite to the direction of the yawing motion of the vehicle is also referred to as a counter-steer (a counter-steering operation). Hereinafter, the steering direction opposite to the direction of the yawing motion of the vehicle is also referred to as a counter-steering direction. More specifically, the counter-steering direction is a direction of steering the steering wheel in a clockwise direction when viewed from a driver in a case where the yawing motion occurs at the vehicle in a direction of turning left (a direction of deflecting to the left). Further, the counter-steering direction is a direction of steering the steering wheel in a counterclockwise direction when viewed from the driver in a case where the yawing motion occurs at the vehicle in a direction of turning right (a direction of deflecting to the right).
Disclosed in JP3034430B is a device for applying a stabilizing force (a stabilizing torque) for inducing or assisting a counter-steering operation, performed by the driver, relative to the steering operation member in the counter-steering direction in a case where the oversteer occurs (i.e. in a case where the counter-steering operation is needed).
A driver skilled in the counter-steering operation (which will be hereinafter referred to as a skilled driver) may actively perform an appropriate counter-steering operation by himself/herself while predicting a behavior of the vehicle in the case where the counter-steering operation is needed, e.g. in the case where the oversteer occurs, in the case where the vehicle deflection, which occurs due to the longitudinal force difference between the right and left wheels, occurs, and the like. Accordingly, for example, in the case where the oversteer occurs while the skilled driver drives the vehicle, to which the device disclosed in JP3034430B is mounted, the skilled driver is likely to perform the counter-steering operation while feeling a lower steering force than a normal (predicted) steering force (an operating torque, a reaction force of the road surface) because of an application of the stabilizing torque. As a result, the skilled driver may feel discomfort when the steering force during the counter-steering operation is reduced contrary to his/her expectation.
A need thus exists to provide a steering control device for a vehicle which is not susceptible to the drawback mentioned above. More specifically, the purpose of the present invention is to provide the steering control device for the vehicle, which applies a stabilizing force (a stabilizing torque) for assisting (inducing) a counter-steering operation performed by a driver relative to a steering operation member in a counter-steering direction when the counter-steering operation is needed, while preventing the driver from having a discomfort feeling that a steering force during the counter-steering operation is reduced contrary to his/her expectation.